1. Technical Field
The present invention relates to cleaning web devices in the fixing station of electrostatographic printing apparatus, such as electrostatic photocopying machines, printers, and facsimile machines; in particular the present invention relates to such cleaning web devices wherein employed in fixing stations configured with auxiliary heating rollers riding on the fixing rollers.
2. Description of Related Art
In electrostatographic printing devices, printing sheets carrying a toner-developed image are conveyed to a fixing station typically having a heating roller against which a pressure roller presses. The toner image is fuse-fixed onto the printing sheets as they are nipped by the pair of rollers in conveyance to the fixing station. In the fixing process, toner particles thus cling to the surface of the heating roller, and consequently a cleaning device is provided in the fixing station for cleaning the heating roller.
Toner that does not get cleaned from the surface of the heating roller may stick to the surface of the pressure roller due to its different separative properties from the heating roller. Further, in double-sided image fixing processes in electrostatographic printing devices, toner may stick to the surface of the pressure roller. Consequently, a cleaning device for the pressure roller is also provided.
Examples of such cleaning devices include unit devices in which a cleaning web that is prepared by impregnating a heat-resistant paper or the like with a parting agent such as silicone oil is stretched from a web feed reel to a web take-up reel, bringing one surface of the cleaning web into contact with the roller surface to be cleaned. The cleaning web is wound at a predetermined rate, and as the roller it contacts rotates, toner sticking to the roller surface is stripped off by the cleaning web.
A problem in thus employing a cleaning web, however, is that in bringing the web into cleaning contact with the roller a major portion of the parting agent contained in the web is applied to the roller surface. Accordingly, if after using one side of a cleaning web, the other side were to be used for another roller cleaning, the amount of parting agent applied to the surface of the roller would be insufficient. Therefore, under the present circumstances only one side of the cleaning web is generally used and then it discarded, which is quite uneconomical.
Meanwhile, among fixing stations, which have a pair of fixing rollers (hereinafter distinctly referred to as a heating roller and a pressure roller, which is pressed against the heating roller), in order to externally heat the surface of that roller directly involved in the thermal fixing process, some are built such that an auxiliary heating roller rides, for example, on the heating roller.
Further, some fixing stations are configured, not only with an auxiliary heating roller riding on the heating roller of the fixing roller pair, but with another auxiliary heating roller riding on the pressure roller. In yet another configuration, the pressure roller alone is provided with an auxiliary heating roller, disposed to ride on the pressure roller surface.
Herein, heating and pressure rollers are selected which are superior in fixing-treated sheet separability, but wherein such rollers are selected, it becomes necessary furthermore to choose an auxiliary heating roller or rollers that is of high sheet separability.
That is, if the auxiliary heating roller is of low sheet separability, toner residual on the surface of the fixing rollers that was not cleaned off by the cleaning web device may get transferred to the surface of the auxiliary heating roller. It therefore becomes necessary to discriminate sheet separability in design selection of the fixing station rollers, taking separability into consideration in selecting the auxiliary roller material as well, so that in operation in an electrostatographic printing device the auxiliary heating roller is not contaminated.
Wherein the heating roller and pressure roller are in view of manufacturing costs relatively inexpensive--fluorocarbon-coated rollers, for example--if an auxiliary heating roller is selected of a material which is of higher sheet separability than the fixing rollers, this brings in the problem of a considerable rise in cost.
In this regard, if the auxiliary heating roller is to be a fluorocarbon-coated roller, one must select a heating roller and a pressure roller which are of lower separability than the auxiliary heating roller in order that is the auxiliary heating roller not be contaminated, giving rise to the problematic point that fixing-treated sheets become difficult to separate from the surfaces of the heating roller and the pressure roller.
Otherwise, if a cleaning device exclusive for the auxiliary heating roller is added, there is no need to for concern regarding its sheet separability in relation to that of the heating and pressure rollers, but this will increase the overall cost of the fixing station by a large margin.